Integrated circuits (ICs) consist of structured electrically semiconducting, non-conducting and conducting thin layers. These patterned layers are customarily prepared by applying a layer material, for example, by vapor deposition and patterning it by a microlithographic process. By way of the combination of the various electrically semiconducting, non-conducting and conducting layered materials the electronic circuit elements such as transistors, capacitors, resistors and wirings are fabricated.
The quality of an IC and of its function depends particularly on the precision with which the various layer materials can be applied and patterned.
However, with an increasing number of layers the planarity of the layers decreases significantly. This leads to the failure of one or more functional elements of the IC and, therefore, to the failure of the complete IC after a certain number of layers has been reached.
The decrease of the planarity of the layers is caused by the buildup of new layers on top of layers already patterned. By the patterning altitude differences are created which can add up to 0.6 μm per layer. These altitude differences add up from layer to layer and bring about that the next following layer can no longer be applied onto a planar surface but only onto an uneven surface. The first result is that the layer subsequently applied has an irregular thickness. In extreme cases, imperfections, defects in the electronic functional elements and lacking electrical contacts are caused. Moreover, uneven surfaces lead to problems with the patterning. In order to be able to create sufficiently small patterns, an extremely acute depth of focus is a necessary in the microlithographic process step. However, these patterns can only be imaged with acuity on a planar surface. The more the locations deviate from the planarity, the murkier the image becomes.
In order to solve this problem, a so-called chemical mechanical polishing (CMP) is carried out. The CMP causes a global planarization of the patterned surface by the removal of protruding features of the layer until a planar layer is obtained. Because of this, the subsequent buildup can take place on top of a planar surface exhibiting no altitude differences, and the precision of the patterning and of the functionality of the elements of the IC is maintained.
Typical examples for the global planarization are dielectric CMP, nickel phosphide CMP and silicium or polysilicium CMP.
In addition to the global planarization to overcome lithographical difficulties, there are two other important applications for CMP. One is to fabricate microstructures. Typical examples for this application are copper CMP, tungsten CMP or shallow trench isolation (STI) CMP, in particular the Damascene process described below. The other is defect correction or elimination, as for example sapphire CMP.
A CMP process step is carried out with the help of special polishers, polishing pads and polishing agents which are also referred to in the art as polishing slurries or CMP slurries. A CMP slurry is a composition, which in combination with the polishing pad causes the removal of the material to be polished.
In case that wafers with semiconductor layers are to be polished, the precision requirements for the process step and, thus, the requirements set for the CMP slurry are particularly strict.
A series of parameters are used for evaluating the efficiency of CMP slurries and for characterizing their activity. The material removal rate (MRR), that is the speed with which the material to be polished is removed, the selectivity, that is the ratio of the removal rate of the material to be polished to the removal rates of other materials present, the removal uniformity within a wafer (WIWNU; within wafer non-uniformity) and the removal uniformity from wafer to wafer (WTWNU; wafer to wafer non-uniformity) as well as the number of defects per unit of area rank among these parameters.
The copper Damascene process is increasingly used for the fabrication of IC (cf., for example, the European patent application EP 1 306 415 A2, page 2, paragraph [0012]). In order to produce the copper circuit paths, it is necessary to remove a copper layer chemically mechanically in this process with the help of a CMP slurry, which process is also called “copper CMP process” in the art. The completed copper circuit paths are embedded in a dielectric. Customarily, a barrier layer is located between the copper and the dielectric.
The CMP agents or slurries customarily used in these CMP processes contain dispersed, colloidal inorganic particles such as silica particles as abrasive materials (cf., for example, U.S. Pat. No. 4,954,142, U.S. Pat. No. 5,958,288, U.S. Pat. No. 5,980,775, U.S. Pat. No. 6,015,506, U.S. Pat. No. 6,068,787, U.S. Pat. No. 6,083,419, and U.S. Pat. No. 6,136,711).
However, there are several disadvantages associated with the use of inorganic particles. First of all, they have to be dispersed in the aqueous phase in a separate dispersion step. Secondly, due to their high densities, they have a tendency to settle out of their aqueous dispersions. Consequently, the respective CMP agents or slurries can be unstable. Thirdly, these CMP agents fail to adequately control dishing and erosion, corrosion, defects of the surface, polishing rate and selectivity among different materials on the surface. Quite often, the inorganic particles and their aggregates cause scratches in the polished surfaces. However, for obvious reasons, such scratches have to be avoided. It must be added that the problem of scratching is particularly severe when the metal wirings or interconnects are embedded in the comparatively soft and spongelike ultra-low-k dielectric materials which are easily damaged by the hard inorganic abrasive particles.
These problems have been ameliorated to a certain degree by the use of organic particles as the abrasive materials.
For example, the European patent application EP 0 919 602 A1 discloses a CMP slurry comprising cross-linked polymeric particles prepared by emulsion polymerization of vinyl compounds having no functional groups that can react with the metal of the surface to be polished, as for example, styrene and divinylbenzene. Optionally, vinyl, acrylate or methacrylate monomers having functional groups such as amide groups, hydroxyl groups, methoxy groups or glycidyl groups may be additionally used. The dispersion resulting from the polymerization can be used directly as a CMP slurry. However, due to the absence or the low concentration of metal reactive functional groups, complexing agents have to be used in order to obtain a high removal rate MRR.
The European patent EP 1 077 240 B1 discloses a CMP slurry comprising cross-linked and non-cross-linked polymeric particles preferably prepared by suspension polymerization of ally unsaturated comonomers using polymerization initiators which introduce reactive functional groups such as amino, pyridyl or acrylamide groups that can react with the metal of the surface to be polished. Also here, the dispersion resulting from the polymerization can be used directly as a CMP slurry. Nevertheless, the concentration of metal reactive functional groups at the surface of the particles needs to be increased, in order to increase the removal rate MRR without increasing the static etch rate SER.
The international patent application WO 2008/148766 A1 discloses highly branched melamine-polyamine condensates. However, only the uses of the melamine-polyamine condensates as catalysts for the polyurethane synthesis, as hardeners for epoxy resins, as DNA transfection agents or as initiators for alkoxylating reactions with ethylene oxide or propylene oxide for the preparation of polyols are described. No mention is made that the melamine-polyamine condensates could be used for the preparation of graft copolymers, which, in turn, could be used in CMP slurries.